1. Field of the Invention
The present invention relates to processes for preparing alpha-hydroxycarboxylic esters on the industrial scale. In particular, the invention relates to a continuous process for preparing alpha-hydroxycarboxylic esters according to the preamble of claim 1.
2. Discussion of the Background
Alpha-hydroxycarboxylic esters are valuable intermediates in the industrial-scale synthesis of acrylic esters and methacrylic esters, referred to hereinafter as alkyl (meth)acrylates. Alkyl (meth)acrylates in turn find their main field of use in the preparation of polymers and copolymers with other polymerizable compounds.
An overview of the common processes for preparing (meth)acrylic esters can be found in the literature, such as Weissermel, Arpe “Industrielle organische Chemie” [Industrial Organic Chemistry], VCH, Weinheim 1994, 4th edition, p. 305 ff. or Kirk Othmer “Encyclopedia of Chemical Technology”, 3rd edition, Vol. 15, page 357.
When the aim is the synthesis of methacrylic esters, for example methyl methacrylate, methyl 2-hydroxyisobutyrate (=MHIB), as the alpha-hydroxycarboxylic ester, is a central intermediate for its preparation.
The preparation of alpha-hydroxycarboxylic esters via the reaction of an alcohol with an alpha-hydroxycarboxamide is detailed by way of example in the publication DE-A-24 54 497. This publication describes the use of lead compounds in order to catalyse the reaction. In this context, mention is also made of continuous processes but without providing a technical solution in which the products are obtained with high efficiency.
Furthermore, the document DE-A-25 28 524 describes processes for preparing alpha-hydroxycarboxylic esters. In this context, various catalysts are used, which include lanthanum compounds among others. Although DE-A-25 28 524 also mentions that the processes described can be performed continuously, this publication also does not provide a satisfactory solution to the problems which occur here.
A process of this type is known from EP 0 945 423. Here, a process for preparing alpha-hydroxycarboxylic esters is disclosed, which comprises the steps of reacting an alpha-hydroxycarboxamide and an alcohol in the presence of a catalyst in a liquid phase, while the ammonia concentration in the reaction solution is kept at 0.1% by weight or less by removing ammonia formed as a gas in a gas phase.
To remove the ammonia from the reaction solution as a gas into the gas phase, it is distilled out of the reaction solution. To this end, the reaction solution is heated to boiling, and/or a stripping gas, i.e. an inert gas, is bubbled through the reaction solution.
The disadvantages of the process disclosed in EP 0 945 423 for the preparation of alpha-hydroxycarboxylic esters by alcoholysis of corresponding alpha-hydroxycarboxamides can be summarized as follows:    i. Simply distilling off the ammonia in a process variant disclosed in EP 0 945 423 is relatively ineffective. The implementation of this proposal requires an extremely effective separating column and hence an exceptional level of technical complexity.    ii. When an inert stripping gas is used additionally or exclusively, the effectiveness of the ammonia removal is improved, but at the expense of a further process component, whose handling means additional complexity.    iii. When alpha-hydroxyisobutyramide and methanol are used as reactants, ammonia and residual methanol formed under the conditions disclosed in EP 0 945 423 can be separated from one another only with very great difficulty.
The fact that it is almost always necessary to use an inert gas for ammonia removal and the associated additional handling of a further stream (stripping gas/ammonia separation) make the procedure proposed economically relatively uninteresting, which is also reflected by the lack of an industrial implementation of the process disclosed to date.